1. Field of the Invention
The present invention relates, in general, to a spacer grid for dual-cooling nuclear fuel rods, which has been developed in order to reduce the core temperature of a nuclear fuel rod, thereby ensuring the safety of the nuclear fuel rod even at super high burnup, and increasing output to obtain economical effects and, more particularly, to a spacer grid for dual-cooling nuclear fuel rods using intersectional support structures, in which the intersectional support structures are fitted into grid straps around intersections of the grid straps in order to compensate for reduction in a gap between the nuclear fuel rod and the support structure due to an increase in diameter compared to an existing nuclear fuel rod, thereby supporting each dual-cooling nuclear fuel rod in a diagonal direction.
2. Description of the Related Art
FIG. 1 is a schematic perspective view illustrating a conventional nuclear fuel assembly. FIG. 2 is a schematic horizontal cross-sectional view illustrating a conventional nuclear fuel assembly. FIG. 3 is a schematic top plan view illustrating a spacer grid, which is applied to a conventional nuclear fuel assembly. FIG. 4 is a schematic perspective view illustrating a spacer grid, which is applied to a conventional nuclear fuel assembly. FIG. 5 is a schematic perspective view illustrating a unit grid strap for a spacer grid supporting a conventional nuclear fuel assembly.
As illustrated in the figures, the nuclear fuel assembly 100 comprises nuclear fuel rods 110, guide pipes 140, spacer grids 150, a top end piece 120, and a bottom end piece 130.
Here, each nuclear fuel rod 110 has a cylindrical uranium sintered compact in a clad pipe of zircaloy (zirconium alloy). This uranium sintered compact is fissioned to produce high-temperature heat.
Meanwhile, each guide pipe 140 is used as a passage for a control rod, which moves up and down in order to control the output of a reactor core and to stop a fission reaction. Each spacer grid 150 is one of the components constituting the nuclear fuel assembly in a nuclear reactor, and is designed so that a spring 118 and dimples 119 of each unit grid strap support the nuclear fuel rods 110 such that the nuclear fuel rods 110 are arranged at designated positions.
When the spring force of the spring 118 and the dimples 119 is too weak, each nuclear fuel rod 110 cannot be supported at a designated position, and thus may not be soundly supported. In contrast, when the spring force of the spring 118 and the dimples 119 is too strong, each nuclear fuel rod 110 undergoes defects, such as scratching, on the surface thereof due to excessive frictional resistance when it is inserted into the spacer grid. Further, during the operation of the nuclear reactor, the nuclear fuel rods 110 experience longitudinal growth by means of the irradiation of neutrons. When this longitudinal growth is not properly accommodated, the nuclear fuel rods 110 undergo bowing.
In this manner, when the nuclear fuel rods 110 undergo bowing, they come nearer to or contact neighboring nuclear fuel rods 110. Thus, a coolant channel, i.e. a sub-channel 115, between the nuclear fuel rods becomes narrow or is blocked. As a result, the heat generated from the nuclear fuel rods is not effectively transmitted to the coolant, thereby increasing the temperature of the nuclear fuel rods. As such, the possibility of generating Departure from Nucleate Boiling (DNB) is increased, which is mainly responsible for the reduction of nuclear fuel output.
The top end piece 120 and the bottom end piece 130 fixedly support the nuclear fuel assembly 100 on upper and lower structures of the reactor core. The bottom end piece 130 includes a filter (not shown) for filtering foreign materials floating in the reactor core.
Meanwhile, each spacer grid 150 is usually made of zircaloy, and includes nuclear fuel rod cells, which support the nuclear fuel rods 110, and guide pipe cells, into which the guide pipes 140 are inserted. Each nuclear fuel rod cell is designed to support each nuclear fuel rod 110 at a total of six supporting points using a total of two grid springs 118, which are located on two respective faces of the nuclear fuel rod cell, and a total of four dimples 119, which are located in pairs above and below the two grid springs 118 and on the other two respective faces.
A cylindrical uranium dioxide compact is charged into each nuclear fuel rod 110, and the coolant rapidly flows from the bottom to the top of the reactor core in the axial direction through sub-channels 115, each of which is enclosed by four nuclear fuel rods 110 or by three nuclear fuel rods 110 and one guide pipe 140.
Here, each sub-channel 115 refers to a space enclosed by the nuclear fuel rods 110, and particularly a passage through which a fluid can freely flow to the neighboring sub-channel because it has an open side.
Meanwhile, as illustrated in FIGS. 6 and 7, a dual-cooling nuclear fuel rod 10 having an annular structure instead of the cylindrical nuclear fuel rod 110 is disclosed in U.S. Pat. Nos. 3,928,132 and 6,909,765.
Here, the dual-cooling nuclear fuel rod 10 having an annular structure includes a sintered compact 11 having an annular shape, an inner clad tube 12, enclosing the inner circumference of the sintered compact 11, and an outer clad tube 13, enclosing the outer circumference of the sintered compact 11. Thus, the coolant flows outside and inside the dual-cooling nuclear fuel rod 10, so that heat transfer is doubled. As a result, the dual-cooling nuclear fuel rod 10 maintains a low surface temperature.
In this manner, in the case in which the dual-cooling nuclear fuel rod 10 is maintained at a low core temperature, the possibility of damaging the fuel due to an increase in the core temperature of the nuclear fuel is reduced, so that the safety margin of the dual-cooling nuclear fuel rod 10 can be increased, and the dual-cooling nuclear fuel rod 10 provides high burnup and high output.
However, in order to make the dual-cooling nuclear fuel rods 10 structurally compatible with an existing pressurized water reactor (PWR) core, the gap between the nuclear fuel rods becomes considerably narrower compared to that between existing nuclear fuel rods because the positions of the guide pipes 140 cannot be changed in the nuclear fuel assembly 100, and because the outer diameter of each nuclear fuel rod is increased. For example, in the case in which the nuclear fuel assembly is formed according to a design draft for the dual-cooling nuclear fuel rods having a 12×12 array, the gap between the nuclear fuel rod and the unit grid strap is reduced from 1.45 mm, which is the size of the existing gap, to about 0.39 mm.
Thus, due to the narrow gap between the nuclear fuel rod and the unit grid strap, it is impossible to use the existing method of forming fuel rod support structures to realize contact at the portions where the fuel rod support structures intersect the nuclear fuel rods in order to support the nuclear fuel rods.